The long term evolution (LTE)/LTE-advanced (LTE-A) or other existing 3rd or 4th generation mobile communication systems utilize a multiple-input multiple-output (MIMO) technique in which transmission is performed using a plurality of transmission/reception antennas in order to increase system capability and data transmission rate. The MIMO technique makes use of a plurality of transmission/reception antennas to spatially separate and transmit a plurality of information streams. The MIMO technique supported by the LTE/LTE-A release 11 and its predecessors supports spatial multiplexing for the case where there are eight transmission antennas and eight reception antennas and supports up to rank-8.
Full dimension-MIMO (FD-MIMO) systems, which have evolved from the legacy LTE/LTE-A MIMO technology, may use eight or more, e.g., 32 or more transmit antennas. In order to effectively implement an FD-MIMO system, the user equipment (UE) needs to exactly measure the channel status and interference magnitude and transmit channel state information based on the exactly measured channel state and interference magnitude to the base station. The base station then determines, e.g., terminals to which the base station sends transmissions, a speed at which the base station sends data, and a precoding it is to apply using the received channel state information. The information fed back from the terminal in LTE/LTE-A-based communication systems generally comes in three types: rank indicator (RI), precoder matrix indicator (PMI), and channel quality indicator (CQI).
The RI, PMI, and CQI are associated with one another and have certain meanings. Different precoding matrixes as supported in the LTE/LTE-A system are defined per rank as an example. Accordingly, although the PMI value when RI is 1 is the same as the PMI value when RI is 2, the PMI values respectively corresponding to the RIs are interpreted in different manners. Further, even when the UE determines the CQI, it assumes that the rank value and PMI value it notified to the base station have been applied by the base station. As such, schemes of generating feedback information assuming a particular transceiver are collectively referred to as implicit feedback. In the implicit feedback scheme, since CQI is generated to include the PMI reported together from the terminal to the base station and the receiver information on the terminal itself, when other cell interference is stable, intra-layer interference may be correctly reflected. That is, the use of implicit feedback benefits that a single user (SU) with rank 2 or higher may enjoy higher-accuracy CQI. By contrast, when the base station uses other precoder than the reported PMI, the CQI loses accuracy and thus the terminal's reception capability is not guaranteed. This means that implicit feedback restricts the freedom for transmission schemes and is not appropriate for multi-user (MU) CQI generation. For this reason, explicit feedback had been discussed until 3rd generation partnership project (3GPP) release 10 was released. In explicit feedback, it is not the case that PMI is generated assuming a particular transceiver, rather it means the dominant eigenvector of the channel. The CQI reported together here may be defined in various meanings. An example is to define the CQI to mean a dominant eigenvalue normalized with interference and noise power. Since explicit feedback generates channel status information without assuming a particular transceiver, it does not limit base station transmission schemes and benefits ease to obtain MU scheduling gain over implicit feedback. However, for the same reason, the CQI in explicit feedback has an inaccurate link adaptation capability as compared with the CQI in implicit feedback generated while interworking with transceiver information. Up to now, LTE/LTE-A MIMO systems primarily support implicit feedback-based SU MIMO for various reasons, such as a limited number of base station antenna ports, inaccurate MU CQI, and feedback costs, and MU MIMO is subject to limited support through rank 1 restriction feedback. By contrast, FD-MIMO system has a significantly higher degree of beamforming freedom over LTE/LTE-A MIMO system designed considering only up to eight one-dimensional array transmit antennas because it has a number of transmit antennas and takes a 2-dimensional antenna array into account. This means FD-MIMO base station may form precise beams for terminals present at different positons, and thus, it is obvious that particularly MU scheduling gain may be anticipated as compared with legacy systems. Meanwhile, as set forth above, use of existing implicit feedback optimized for SU MIMO would give only a limited MU scheduling gain. Thus, a need exists for a feedback scheme that may maximize the MU scheduling gain while supporting legacy terminals.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present disclosure.